This invention relates to a reinforcement structure for an inner layer sheet made of a porous dielectric material to be used in multilayer units, such as multilayer printed circuit boards or multilayer printed wiring boards, formed by etching wiring patterns in metal sheets, or by laminating several metal sheets.
Heretofore sheets used for the inner layers of laminated metal sheets and laminated printed circuit boards made using heat-curable materials often had a high dielectric constant. More specifically, inner layers of multilayer structures formed of a porous dielectric material have been proposed, especially in cases where it is necessary to satisfy such requirements as shortening of time delay in transmission of signals between printed circuits in multilayer printed circuit boards made of the above mentioned laminated metal layers, or having etched patterns, reduction in electrostatic capacity, reduction of space between the wiring elements to provide for an increase in density of the elements, etc.
However, the above mentioned internal layer sheets made from porous dielectric material can be easily crushed in the thickness direction because of the porous nature of the electric material. This crush is especially noticeable when the above mentioned porous dielectric material has continuous pores. In that case, dimensional stability of the inner layer in the thickness direction is considerably impaired. When damage occurs in the inner layer sheet, printed circuits formed in this sheet experience problems associated with the above mentioned requirements, such as shortening of signal transmission delay time, etc.